1. Field of the Invention
The present invention generally relates to wedge-threaded components of tubular connections. More particularly, the present invention relates to wedge-threaded components of tubular connections incorporating a metal-to-metal seal therebetween. More particularly still, the present invention relates to single-step multi-portion wedge-threaded tubular connections incorporating high-angle metal-to-metal seals.
2. Background Art
Casing joints, liners, and other oilfield tubulars are frequently used to drill, complete, and produce wells. For example, casing joints may be placed in a wellbore to stabilize and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could otherwise damage the formation. Casing joints are sections of pipe (e.g., steel or titanium), which may be coupled in an end-to-end manner by threaded connections, welded connections, or any other connection mechanisms known in the art. As such, connections are usually designed so that at least one seal is formed between an interior of the coupled casing joints and the annulus formed between exterior walls of the casing joints and the interior walls of the wellbore (i.e., the formation). The seals may be elastomeric (e.g., an o-ring seal), thread seals, metal-to-metal seals, or any other seals known to one of ordinary skill in the art.
It should be understood that certain terms are used herein as they would be conventionally understood, particularly where threaded tubular joints are connected in a vertical position along their central axes such as when making up a pipe string for lowering into a well bore. Typically, in a male-female threaded tubular connection, the male component of the connection is referred to as a “pin” member and the female component is called a “box” member. As used herein, “make-up” refers to engaging a pin member into a box member and threading the members together through torque and rotation. Further, the term “selected make-up” refers to the threading of a pin member and a box member together with a desired amount of torque or based on a relative position (axial or circumferential) of the pin member with respect to the box member. Furthermore, the term “box face” is understood to be the end of the box member facing outward from the box threads and the term “pin nose” is understood to be the end of the pin member facing outward from the threads of the connection. As such, upon make-up of a connection, the nose of the pin is stabbed or inserted into and past the face of the box.
Referring to the geometry of threads, the term “load flank” designates the side wall surface of a thread that faces away from the outer end of the respective pin or box member on which the thread is formed and supports the weight (i.e., tensile load) of the lower tubular member hanging in the well bore. Similarly, the term “stab flank” designates the side wall surface of the thread that faces toward the outer end of the respective pin or box member and supports forces compressing the joints toward each other such as the weight of the upper tubular member during the initial make-up of the joint or such as a force applied to push a lower tubular member against the bottom of a bore hole (i.e., compressive force).
One type of threaded connection commonly used in oil country tubular goods is a wedge thread. Referring initially to FIGS. 1A and 1B, a prior art tubular connection 100 having a wedge thread is shown. As used herein, “wedge threads” are threads, regardless of a particular thread form, that increase in width (i.e., axial distance between load flanks 225 and 226 and stab flanks 232 and 231) in opposite directions on a pin member 101 and a box member 102. The rate at which the threads change in width along the connection is defined by a variable known as the “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the width of the threads to vary along the connection. Furthermore, as used herein, a thread “lead” refers to the differential distance between a component of a thread on consecutive threads. As such, the “stab lead” is the distance between stab flanks of consecutive thread pitches along the axial length of the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436, issued to Mallis, assigned to the assignee of the present invention, and incorporated by reference in its entirety herein. Furthermore, wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Referring still to FIGS. 1A and 1B, a pin thread crest 222 in a wedge thread coupling is narrow towards a distal end 108 of pin member 101 while a box thread crest 291 is wide. Moving along an axis 105 (from right to left), pin thread crest 222 widens while box thread crest 291 narrows as it approaches a distal end 110 of box member 102. As shown in FIG. 1A, the threads are tapered, meaning that a pin thread 106 increases in diameter from beginning to end while a box thread 107 decreases in diameter in a complimentary manner. Having a thread taper may improve the ability to stab pin member 101 into box member 102 and distribute stress throughout the connection.
Generally, thread seals are difficult to achieve in non-wedge (i.e., free-running) threads. However, thread forms that are unable to form a wedge seal in a free-running configuration may create thread seals when used in a wedge thread configuration. As should be understood by one of ordinary skill, as wedge threads do not require any particular type or geometry of thread form, a variety of thread forms may be used. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239, issued to Klementich and incorporated herein by reference in its entirety. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706, issued to Church and incorporated herein by reference in its entirety. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e., not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880, issued to Watts and incorporated herein by reference in its entirety. As such, the above thread forms (i.e., those of Klementich, Church, and Watts) are examples of thread forms that may be used with embodiments of the invention. Generally, open thread forms such as buttress or stub are not suitable for wedge threads, as they would impart a large radial force on the box member. However, a generally square thread form, such as that disclosed by Watts, or a trapped thread form, may be used, as they do not impart an outward radial force on the box member. As such, those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
Referring again to FIGS. 1A and 1B, in wedge threads, a thread seal may be accomplished through contact pressure caused by interference that occurs at make-up over at least a portion of connection 100 between pin load flank 226 and box load flank 225 and between pin stab flank 232 and box stab flank 231. Close proximity or interference between roots 292 and 221 and crests 222 and 291 complete the thread seal when occurring proximate to such flank interference. Generally, higher pressures may be contained either by increasing interference between the roots and crests (“root/crest interference”) on pin member 101 and box member 102 or by increasing the aforementioned flank interference.
Although various wedge thread connections exist having positive-stop torque shoulders (e.g., Klementich, referenced above), wedge threads typically do not have torque shoulders, so their make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member may vary more during make-up for a given torque range to be applied than for connections having a positive-stop torque shoulder. For wedge threads designed to have flank interference and root/crest interference at a selected make-up, the connection is designed such that both the flank interference and root/crest interference increase as the connection is made-up (i.e. an increase in torque increases flank interference and root/crest interference). For tapered wedge threads having root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks come closer to each other (i.e., clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection. However, increased interference and make-up torque may increase stress on the connection during make-up, which may lead to premature failure of the connection.
Furthermore, as shown, connection 100 includes a metal-to-metal seal 112 created by contact between corresponding seal surfaces 103 and 104, respectively located on pin member 101 and box member 102. Metal-to-metal seal 112 provides an additional measure of seal integrity (i.e., when a wedge thread seal is not sufficient) for threaded connection 100, and is particularly useful where connection 100 is intended to contain high-pressure gases. While the metal-to-metal seal is shown located proximate to the distal end 108 of pin member 102, it should be understood by one of ordinary skill in the art that metal-to-metal seal 112 may be positioned anywhere along the length of connection 100, including, but not limited to, a location proximate to the distal end of box member 102.
Nonetheless, seal surfaces 103 and 104 of metal-to-metal seal 112 are usually constructed as corresponding frusta-conical surfaces characterized by a low angle (e.g., an angle less than about 4 or 5 degrees) of intersection with their corresponding remaining pin 101 and box 102 surfaces. Typically, low-angle metal-to-metal seal surfaces 103 and 104 are used in conjunction with wedge thread connections (e.g., 100) because the indeterminate make-up thereof necessitates a seal capable of less precise axial alignment. As wedge threads make-up indeterminately, the relative axial position of pin member 101 and box member 102 will vary over successive make-up and breakout cycles. However, one disadvantage of a low-angle metal-to-metal seal is that seal surfaces 103 and 104 have larger friction contact areas than higher-angle seals, and as such, have less resistance to galling upon make-up. Furthermore, as low-angle metal-to-metal seals engage slowly (i.e., low radial displacement per revolution), the seals must be in contact for several revolutions. As such, in a wedge thread connection including a metal-to-metal seal, the seal is typically the first thing to “make-up,” such that the initial engagement of the seal marks the “hand tight” state of such a threaded connection. Therefore, while low-angle seals are beneficial in that they accommodate the indeterminate make-up characteristics of wedge threads, they may become ineffective over repeated make-up and break-out cycles as seal surfaces 103 and 104 are deformed and/or are cold-worked out of specification.
In contrast, free-running threads used in oilfield tubular connections typically do not form thread seals when the connection is made-up. Referring now to FIG. 2, a prior art connection 200 having free-running threads is shown. The free-running threads include load flanks 254 and 255, stab flanks 257 and 258, crests 259 and 262, and roots 260 and 261. As is typical of a connection with free-running threads, connection 200 relies on a positive-stop torque shoulder formed by the contact of surfaces 252 and 251 disposed on a pin member 201 and a box member 202, respectively. The positive-stop torque shoulder shown in FIG. 2 is commonly referred to as a “pin nose shoulder.” In other connections, the positive-stop torque shoulder may instead be formed by a box face 263 and a mating shoulder (not shown) on pin member 201. The positive-stop torque shoulder also provides a seal. Unlike wedge threads (e.g., those shown in FIG. 1B), which make-up by the wedging of the pin thread 106 and the box thread 107, free-running threads rely on the positive-stop torque shoulder to load connection 200 during make-up. To make-up connection 200, pin member 201 and box member 202 are screwed together until surfaces 251 and 252 are brought into abutment, at which point pin load flank 254 and box load flank 255 are also in abutment. Additional torque is applied to pin member 201 and box member 202 to load surfaces 252 and 251 and pin load flank 254 and box load flank 255 until the desired amount of make-up torque (i.e., the selected make-up) has been applied to connection 200.
Because a large gap 253 exists between pin stab flank 257 and box stab flank 258, connection 200 does not accomplish a thread seal. Gap 253 occurs as a result of how free-running threads with positive-stop torque shoulders are loaded. Applying torque to connection 200 during make-up against the positive-stop torque shoulder causes pin member 201 to be compressed while box member 202 is stretched in tension. Note that when a box face shoulder is used, box member 202 is compressed while pin member 201 is stretched in tension. The force between pin member 201 and box member 202 is applied through pin load flank 254 and box load flank 255. Notably pin stab flank 257 and box stab flank 258 are not loaded during make-up. This results in contact pressure between load flanks 254 and 255 and gap 253 between stab flanks 257 and 258. As discussed above, a wedge thread (e.g., FIG. 1) forms a thread seal in part because of the interference between load flanks 225 and 226 and stab flanks 232 and 231. In wedge threads, this occurs near the end of the make-up of the connection because of the varying width of pin thread 106 and box thread 107. To have similar interference between load flanks 254 and 255 and stab flanks 257 and 258 on a cylindrical (i.e., non-tapered) free-running thread, the interference would exist substantially throughout the make-up of the connection because pin thread 206 and box thread 207 have a continuous width. Furthermore, root/crest interference, if any, would exist substantially throughout the make-up of the connection. This could lead to galling of the threads and difficulty in making-up the connection.
The variance in thread width for a wedge thread occurs as a result of the load flanks having different leads than the stab flanks. A thread lead may be quantified in inches per revolution or by the commonly used inverse term “thread pitch,” (i.e., threads per inch). A graph of the leads for a prior art wedge thread is shown in FIG. 3A. For this connection, the load lead 14 is constant over the length of the connection and greater than the stab lead 12, which is also constant. The nominal lead is shown as item 10. As used herein, “nominal lead” refers to the average of load lead 14 and stab lead 12. The thread will widen with each revolution by the difference in load lead 14 and stab lead 12. As described above, the difference in the load lead 14 and the stab lead 12 is the “wedge ratio.” For a free-running thread (i.e., non-wedge thread), load lead 14 and stab lead 12 would be substantially equal, thus causing the free-running thread to have a substantially constant thread width (i.e., a zero wedge ratio).
Intentional variances in thread leads have been disclosed in the prior art for the purposes of load distribution. One example of a varied thread lead for stress distribution is disclosed in U.S. Pat. No. 4,582,348 issued to Dearden, hereby incorporated by reference in its entirety. Dearden discloses a connection with free-running threads that has the pin thread and box thread divided into three sections, each with different leads (note that Dearden refers to thread pitch, which is quantified as threads per inch). In FIG. 3B, a graph of the thread leads for the box member and the pin member of Dearden is shown. As shown in the graph, at one end of the connection, a pin thread lead 21 is larger than the box thread lead 22. In the intermediate section 23, the pin thread lead 21 and box thread lead 22 are substantially equal. Then, at the other end of the connection, box thread lead 22 is larger than pin thread lead 21. In Dearden, the changes in pin thread lead 21 and box thread lead 22 are step changes (i.e., substantially instantaneous changes in the lead). The varied thread leads disclosed by Dearden are intended to distribute loading across a greater portion of the connection, and have no effect on the ability of the free-running threads to form a thread seal. Dearden does not disclose varying a load lead or stab lead independent of one another.
Another connection is disclosed in U.S. Pat. No. 6,976,711, issued to Sivley, assigned to the assignee of the present invention, and hereby incorporated by reference in its entirety. Sivley discloses connections having a variance in load lead and/or stab lead on one or both of the pin member and the box member. A graph of an embodiment disclosed by Sivley is shown in FIG. 3C. Sivley discloses varying load lead 14 relative to stab lead 12 at a selected rate over at least a portion of the pin thread and/or box thread. In FIG. 3C, the connection is a wedge thread as shown by the difference between load lead 14 and stab lead 12. Load lead 14 and stab lead 12 converge at a linear rate towards the end of the thread. Sivley discloses various other embodiments having load leads 14 and stab leads 12 that vary at linear rates relative to each other. The variance in the thread leads distributes the loads experienced by the connection over the length of the connection.
Referring now to FIG. 4, a prior art two-step connection is shown. The threads that form the connection are separated across multiple “steps,” a large step, indicated by a bracket 31, and a small step, indicated by a bracket 32. The portion between large step 31 and small step 32 is commonly referred to as a mid-step 33. In some connections, mid-step 33 may be used as a metal-to-metal seal. Preferably, a pin thread crest (often referred to as a major diameter in a non-tapered threaded connection) on small step 32 of pin member 401, at its full design height, does not interfere with a box thread crest (often referred to as a minor diameter in a non-tapered threaded connection) on large step 31 of box member 402 when pin member 401 is stabbed into box member 402. The diameter of small step 32 of pin member 401 is smaller than the smallest crest-to-crest thread diameter on large step 31 of box member 402 so a pin thread 406 on small step 32 may be stabbed past a box thread 407 on large step 31. The threads on both small step 32 and large step 31, which have substantially the same nominal lead, engage with each revolution to make-up the connection. Thus, the number of revolutions during which the threads slide or rub against each other is reduced for the same number of engaged threads. A multi-step connection allows for each of the steps to have threads with different characteristics as long there is little or no variance in the nominal lead of the threads on the steps.
A two-step wedge thread connection is disclosed in U.S. Pat. No. 6,206,436, issued to Mallis, and hereby incorporated by reference herein. Mallis discloses a two-step wedge thread connection having different wedge ratios, one of which is considered to be an aggressive wedge ratio and the other a conservative wedge ratio, wherein “aggressive” refers to a larger wedge ratio, and “conservative” refers to a smaller wedge ratio. With all other thread characteristics held constant, a greater wedge ratio will exhibit a more determinate make-up. However, too large of a wedge ratio may have an inadequate wedging effect, which may allow the connection to back off during use. Conversely, smaller wedge ratios are better able to resist backing-off of the connection, but may have such an indeterminate make-up that galling may occur over the lengthened make-up distance. Mallis discloses that one of the steps in a multi-step thread may have a wedge ratio optimized for a more determinate make-up (aggressive), while another step may have a wedge ratio optimized to prevent backing-off of the connection (conservative).
U.S. Pat. Nos. 6,174,001 and 6,270,127 issued to Enderle, assigned to the assignee of the present invention, and incorporated by reference herein, disclose two-step, low torque wedge threads for tubular connections. In the references, one step is provided so that there is interference at make-up along at least one of the complementary stab flanks, load flanks, roots, and crests while clearance is provided along another step along at least one of the complementary stab flanks, load flanks, roots, and/or crests.
This configuration reduces the amount of torque required for make-up of the connection while retaining torque sensitivity, sealing capability, and threads necessary for structural purposes.
One problem with two-step connections is that the connection must be thick to reach 100 percent pipe body efficiency. As used herein, “pipe body efficiency” is the tensile strength of the connection relative to the tensile strength of the tubular. The primary reason for needing a thicker connection is the unengaged space of the mid-step, which is required so that the threads on the large step may clear the threads on the small step during stabbing. The mid-step, due to the lack of thread engagement, does not contribute to the overall strength of the connection. The advantages of having two separate threads often makes up for the decreased pipe body efficiency, however, it is desirable to have a single-step thread that may exhibit the advantages of two-step connections.